February 18, 2020

Development and Progress of Instrument Transformer

Transformer comprehensive tester manufacturers traditional electromagnetic CT is mainly divided into protective CT and measurement CT. The protection type CT mainly tests its volt-ampere characteristics, 10% error curve, and transformation ratio, polarity, etc.; measurement-type CT mainly tests its transformation ratio, polarity, ratio difference, angle difference and various primary currents, various kinds of The indicators such as the ratio of difference and angle difference at the time of the secondary load.
First, the traditional power frequency voltage and current method to test CT voltage characteristics and change ratio method
The traditional domestic test methods for the protection of CT volt-ampere characteristics are mainly manual pressure adjustment measurement methods. The test principle is shown in the figure below. The output voltage of the auto-regulator regulator, which is manually adjusted, is added to the CT unit, and readings are made using a common voltage and ammeter. Two people operate, one person operates the regulator and the other reads. When reading the voltage booster up to a certain value, read the voltage and current data and manually plot the volt-ampere characteristic curve.
The principle of variable ratio measurement is shown in the figure below. By manually adjusting the output voltage of the voltage regulator and converting it into a large current by the current transformer to the original CT side, measuring the current of the payer, and calculating the ratio of the current of the original payor, the variable ratio value can be calculated.
The main drawbacks of the traditional method are the complicated wiring, the large amount of labor, the slow speed, the poor measurement accuracy, and the error of tolerance. Test volt-ampere characteristics sometimes need to add up to 2000V voltage, so manual operation is very poor.
The traditional method is obviously very backward and needs to be improved.
Second, the test methods and test instruments for CT have undergone three stages of changes
1. First-generation instruments: semi-automatic (manual pressure regulation, automatic measurement) CT volt-ampere characteristics
Based on the basic principle of the traditional direct measurement method, a measuring device for a digital recording method was developed. The manual voltage regulator and the digital voltage and current measurement part and the single-chip circuit are installed in the same device, and the voltage is manually adjusted, the microcontroller records the data, and the volt-ampere characteristic curve is depicted. This method has achieved a bit of technical advancement over the full manual test method, but it is inconvenient to operate and the safety and reliability are still not high.
The main disadvantages of transformer tester :
● Manual operation, slow
● Measurement accuracy is not high
● Due to manual direct adjustment, operation safety is extremely poor
● Directly generate large voltage currents for measurement, large pressure regulators, boosters, and clumsiness.
2. Second-generation instruments: automatic type, auto-boost method to test CT volt-ampere characteristics, and automatic up-flow test to measure the ratio of CT to polar
Microprocessor controlled automatic test device. The built-in stepper motor controls the automatic voltage booster to generate the voltage, automatically measure and depict the volt-ampere characteristic curve, and calculate the 10% error curve. The built-in automatic current transformer measures the CT ratio and polarity. Test principle is shown in the figure below.
main feature:
● Fully automatic test: It can automatically test the CT's volt-ampere characteristic, ratio, polarity, secondary load and 10% error curve, simplifies test wiring, and has high test efficiency.
● Higher accuracy measurement: Eliminate measurement errors caused by manual readings and manual processing.
● High security: The wiring is simple, the test process is automatic, no manual contact is needed, and high security is ensured.
● Automatic processing of measurement results: Real-time display of test data and plotting of curves, calculation of inflection points, automatic storage and printing, and upload of computer archives.
The main drawbacks:
● It still belongs to the direct measurement method of power frequency, and it needs to generate high voltage and high current for testing.
● Due to the use of large pressure regulators and boosters, large volume and weight
● Because of the wide range of voltage and current to be measured, the measurement accuracy cannot be made very high. Therefore, it can only be used to test protective CT, and measurement CT cannot be tested.
Typical representative:
3rd, third generation instrument: The low frequency method tests the volt-ampere characteristic, the voltage method tests the transformation ratio and the polarity, may cover the complete general test that protects the CT and the measurement CT
The principle of the low-frequency method to test the volt-ampere characteristic: In the case of the same magnetizing inductance of the CT, the excitation impedance value is proportional to the frequency. Therefore, to achieve the same saturation of the magnetizing inductance, the lower the frequency of the applied voltage is, the lower the amplitude of the voltage is. This is the basic principle of the low-frequency method to test the volt-ampere characteristic. Low-frequency measurement can reduce the applied voltage amplitude (ie, reduce the power of the applied voltage), so as to achieve a small size, light weight, and greatly improve the measurement accuracy, can be used to protect the CT volt-ampere characteristics can also be used to measure the CT ratio difference Angle difference measurement. This method can achieve full and complete testing of all CTs.
The voltage method measures the principle of transformation ratio: when AC voltage is applied to the secondary winding of the CT, an induced voltage will be generated on the primary side, and the ratio of the AC voltage on the secondary winding core to the amplitude of the primary side induced voltage is theoretically equal to the turns ratio, and Compared with the direct method in which a large current passes through the side, this ratio test method does not require a large current, and has the characteristics of small test equipment capacity, safety, and reliability. This CT ratio test method is also called indirect method.
Transformer comprehensive tester main features:
●Comprehensive function: Can be used for various types of CT (including TP) volt-ampere characteristics, 10% error curve, transformation ratio, phase, polarity, secondary winding resistance, secondary loop load, ratio difference and angle difference, and Steady state or transient characteristics such as ratio difference and angle difference under different operating currents and different load conditions
● Measurement and verification of various types of CT, including protection CT, measurement CT, TP transient CT, excitation saturation voltage of 30KV CT, transformer casing CT
●Overall comprehensive test covering CT and metrology CT, including TP transient CT, CT with saturated voltage up to tens of thousands volts, transformer casing CT, etc. This is the technical and application advantage that cannot be matched by the direct power frequency method.
● High-precision measurement: Due to the small measurement voltage range, high measurement accuracy can be achieved with an accuracy of 0.05%
● Only need to output smaller voltage and power, so small, light weight, light and small portable
● Automatic processing of measurement results: Real-time display of test data and plotting of curves, automatic storage and printing. And can upload computer management and archiving.
The main drawbacks:
● Because the direct method is used to measure, no large current is output, so it cannot be used as a high-current source for high-current output of the field.
Some types of instruments do not have a high-precision measurement function. Therefore, they can only be used to protect the CT's volt-ampere characteristics and the polarity ratio of the CT. They can not be used for differential and angular difference measurements, and cannot be used to measure CT. Test
Typical representative: CT parameter analyzer
Third, the basic principle of frequency measurement method
The principle circuit of the CT volt-ampere characteristic measurement is as follows: CT is open on the primary side, voltage is applied from the secondary side, and the relationship between the applied voltage V and the input current I is measured. This curve approximates the relation between the excitation potential E of the CT and the excitation current I.
Assuming that the excitation inductance of the CT excitation winding at a certain excitation current I is L and the excitation impedance is Z, then:
V = I·Z
The relationship between the inductance L and the impedance Z has the following relationship:
Z = ω·L = 2 π f L
Then: V = I·2 π f L
It can be seen from the formula that the voltage V applied at a certain magnetizing inductance L is proportional to the frequency f.
Assume that when f = 50hz, in order to reach the magnetizing current Ix, the required applied voltage Vx is 2000V
Vx = I·2 π f L= Ix·2 π f L = 2000V,
If different frequencies are applied:
f = 50hz, Vx = 2000V
f = 5hz, Vx ≌ 200V
f = 0.5hz, Vx ≌ 20V
It can be seen that the need to make the CT into the same degree of saturation, the voltage required to apply a lower frequency signal can be greatly reduced This is the basic principle of the frequency conversion method.
It must be rigorously noted here that the required voltage is not linearly proportional to the frequency, and that instead of decreasing in proportion to the frequency, a complete theoretical calculation must be performed in strict accordance with the precise mathematical model of the transformer.

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